1. Field of the Invention
The present invention relates to an electron gun and an electron beam exposure apparatus. Particularly, the present invention relates to an electron gun and an electron beam exposure apparatus which are suitable for saving the consumption of electron gun materials.
2. Description of the Related Art
Recent electron beam exposure apparatuses are configured to perform transfer exposure as follows to improve their throughputs. Specifically, each electron beam exposure apparatus is equipped with a mask having a variable rectangular opening or multiple mask patterns, and performs transfer exposure onto a wafer by selecting a desirable rectangular opening or mask pattern through beam deflection. An electron beam exposure apparatus making a cell projection exposure has been proposed as a method of making an exposure by use of multiple mask patterns. A cell projection exposure apparatus transfers a pattern onto a workpiece as follows. The cell projection exposure apparatus applies an electron beam onto a pattern area selected, through beam deflection, from multiple patterns arranged in a mask, and thereby forms the cross-section of the beam into a shape of the selected pattern. Then, the beam having passed the mask is again deflected and thus swung back by a deflector installed in its subsequent stage, and is reduced in size at a certain reduction ratio determined by its electro-optical system. Thereby, the pattern represented by the beam is transferred onto the workpiece.
In the exposure apparatus of this type, the securing of the line width precision is also important for the throughput improvement. To secure the line width precision, it is required that the strength of the electron beam emitted from the electron gun should not change with time. If the strength of the electron beam becomes weaker over time, the exposure degree gradually decreases. The exposure time may be extended to compensate for the decreased exposure degree. However, such exposure time extension would not only require a complicated control, but also reduce the throughputs.
In general, methods for causing an electron gun to emit electrons are roughly classified into a thermal electron emission type and a field emission type. An electron gun of the thermal electron emission type includes: a cathode for emitting electrons when heated; a Wehnelt cylinder for creating an electron flux by converging the electrons emitted from the cathode; and an anode for accelerating the converged electron flux.
The following phenomenon occurs when the thermal electron emission type (also referred to as “thermal field emission type) electron gun is used. Specifically, an electron emission portion of the electron gun changes its shape as an electron source (tip) used for the electron gun emits electrons. That is because a substance constituting the tip sublimes, evaporates, and thereby decreases in amount. Various countermeasures against this phenomenon have been put under examination. For instance, Japanese Patent Application Publication No. Hei 8-184699 discloses an electron gun including a tip whose surface is covered with a double-layered film consisting of a tungsten (W) layer and a rhenium (Re) layer so that the tip is consumed less than ever.
As described above, when the thermal electron emission type electron gun is used, the tip constituting the electron gun not only emits electrons, but also allows the substance constituting the tip itself to sublime in some cases. It is considered that the sublimation of the tip occurs because, when electrons are emitted by thermal electron emission, the tip is heated at a temperature higher than a sublimation temperature of the electron generating substance.
This sublimation changes the shape of the tip for emitting electrons. This change inhibits a uniform irradiation of the beam of the variable rectangle and the beam of the cell projection pattern, and decreases the strength of the emitted electron beam. For instance, in the case of a thermal electron emission type electron gun which includes a tip made of lanthanum hexaboride (LaB6), and which heats the tip at 1500° C., 10 μm of lanthanum hexaboride sublimed in one month use.
The tip substance, for instance, LaB6 or cerium hexaboride (CeB6), adheres to the back of the grid through the sublimation. It is likely that this adhering substance may turn into whiskers, which may be charged with electrons and then discharge in minute scale. Once the micro discharge takes place in this manner, a phenomenon occurs in which the amount and irradiation position of an electron beam become unstable. This phenomenon hinders the normal use of the electron beam exposure apparatus, and also decreases the throughputs because it takes time for adjustment or the like. The most serious problem is a loss of the reliability of a pattern written while the micro discharge occurs. Accordingly, for enhancing the reliability of the electron beam exposure apparatus, it is essential to completely prevent the minute electrical discharge from occurring in the vicinity of the electron gun. In sum, the reduction in the amount of sublimation of the material for the electron gun is a prerequisite condition for developing a highly-reliable and highly-stable electron gun.
The electron beam exposure apparatus adopts a Koehler's illumination mode to make an even exposure on a mask pattern on the workpiece. The Koehler's illumination mode causes a crossover image of an electron beam to be formed on the pupil of the final lens. In the field emission type electron gun, for example, the virtual crossover image is as small as approximately 2 μm to 3 μm in size. This small crossover image decreases the illuminance and the electric current density, and eventually causes a problem of reduced exposure throughputs.
In addition, this small crossover image increases an angle at which the beam is emitted from the electron gun. As a result, it is likely that: a number of electrons may collide against the accelerating electrode which raises the temperature of the accelerating electrode; and the raised temperature may decrease the vacuum degree.
Furthermore, the electron gun disclosed by the Japanese Patent Application Publication No. Hei 8-184699 noted above reduces the consumption of its tip by covering the tip with the double-layered film consisting of the tungsten layer and the rhenium layer. However, the electron gun in this example is still incapable of preventing its electron emission surface from changing its shape through sublimation because the electron emission surface is not covered by the double-layered film.